We did a desperate upgrade to CURRENT in hope that this would fix the problem ;)
The machine became much faster but as soon as we put some load on, it panics but
still responds to ping. There is nothing in the logs after a power cycle.
Is there anything we can do to help troubleshoot this, would a DEBUG-kernel
help? We can supply ssh access if that would make it easier.
The load that causes this is a MySQL import over the network, a lot of CPU, disk
access and nfs. The machine panics after 30-60 minutes into the import.

Do you have the panic message, and eventually the stack trace ?

We don't have the panic message (no console access) but we are trying to
create an usable core dump.

When we used a MP kernel gdb complained about "Cannot access memory at
address 0xffff80005bfca6c0" so right now we have booted on one CPU and
are trying to produce a panic, but so far it is stable. Before when we
copied files over NFS it paniced several times a few GB into the copy
but now it runs fine, perhaps this has to do with the MP-kernel?